An 18-year-old man was stabbed to death and another man was knifed in the leg just hours later on London's Oxford Street, which was packed with thousands of shoppers out for the opening day of the Boxing Day sales.

Police cordoned off a 300-metre section of the crowded thoroughfare during the afternoon on one of the busiest days in the retail year, forcing several stores to close.

Panicked shoppers were ushered to safety as armed police rushed to the scene after reports of a knife attack. Witnesses described seeing a black teenager lying in a pool of blood as paramedics battled to save him.

Ten people were arrested across London in connection with the killing, which occurred either inside or just outside the flagship branch of the Foot Locker sportswear chain at about 1.45pm. It was reported that at least one of the people arrested was also injured.

At about 6.30pm a 21-year-old man was stabbed in the thigh on the corner of Regent and Oxford streets – a few hundred metres away from the first incident. The man was receiving treatment in hospital last night, but police described his condition as "stable".

Asked whether the two stabbings were linked, Inspector Bruce Middlemiss, of Westminster Police, said: "There's nothing firm; however, there are similar circumstances – youths possibly from the same south London area."

Commenting on the first stabbing, the senior investigating officer, Detective Chief Inspector Mark Dunne, said an "assortment of weapons" had been recovered from the scene, but it was too early to say whether the incident was gang related.

A photographer had earlier taken a picture of a blood-smeared knife found nearby, but police could not confirm the weapon was linked to the killing.

Det Ch Insp Dunne said: "This is the busiest place in the UK right now and it has been difficult to piece together what happened. The flip side is we have more witnesses than normal."

It is believed an argument flared between two groups of young people who were inside the Foot Locker store and quickly escalated into a fight.

The victim of the fatal stabbing, from south London, collapsed just outside the shop where he was treated by the London Ambulance Service. Paramedics were unable to save him and he was declared dead at the scene. Everyone inside the shop at the time was being questioned by police.

As the Christmas lights came on around the capital's most popular shopping district last night, forensic officers erected a tent outside the shop.

Among the shops forced to close were the Disney Store and Boots. The Bond Street Tube station and several doors to Debenhams were also within the cordon, which ran from New Bond Street to Duke Street near Selfridges.

It is not the first time that Oxford Street has been the scene of violence. Steven Bigby, 22, from Hackney, was killed in broad daylight outside a McDonald's restaurant after a drink was thrown in 2008. Anthony Costa, 19, of Walthamstow, east London, was jailed at the Old Bailey in December that year for the attack and told he would serve at least five years.

In August, two teenagers were stabbed outside an H&M shop on Oxford Street in front of hundreds of shoppers.

Insp Middlemiss said the general public should not feel unsafe shopping in the West End despite the day's violence: "There is a high police presence here. We have a number of operations in place already for the shopping times because we were aware of the amount of people who would be here.

"We are going to review the intelligence for both incidents and see if we need to increase the police presence here, but at the moment we are not concerned for ordinary members of the public who are shopping," he said.